Transmission Electron Microscopy Measurements Research Articles

Ultrahigh Energy Storage in (Ag,Sm)(Nb,Ta)O3 Ceramics with a Stable Antiferroelectric Phase, Low Domain-Switching Barriers, and a High Breakdown Strength.

AgNbO3 (AN) antiferroelectrics (AFEs) are regarded as a promising candidate for high-property dielectric capacitors on account of their high maximum polarization, double polarization-electric field (P-E) loop characteristics, and environmental friendliness. However, high remnant polarization (Pr) and large polarization hysteresis loss from room-temperature ferrielectric behavior of AN and low breakdown strength (Eb) cause small recoverable energy density (Wrec) and efficiency (η). To solve these issues, herein, we have designed Sm3+ and Ta5+ co-doped AgNbO3. The addition of Sm3+ and Ta5+ reduces the tolerance factor, polarizability of B-site cations, and domain-switching barriers, enhancing AFE phase stability and decreasing hysteresis loss. Meanwhile, adding Sm3+ and Ta5+ leads to decreased grain sizes, increased band gap, and reduced leakage current, all contributing to increased Eb. As a benefit from the above synergistic effects, a high Wrec of 7.24 J/cm3, η of 72.55%, power density of 173.73 MW/cm3, and quick discharge rate of 18.4 ns, surpassing those of many lead-free ceramics, are obtained in the (Ag0.91Sm0.03)(Nb0.85Ta0.15)O3 ceramic. Finite element simulations for the breakdown path and transmission electron microscopy measurements of domains verify the rationality of the design strategy.

Enhanced properties of hard metal WC-Ni processed from nanostructured powders

In this study, we investigated the effects of nanostructured composite powders (WC-Ni), nickel binder content, and sintering techniques on the morphology and mechanical properties of tungsten carbide. An alternative low-temperature gas-solid reaction route with a short reaction time was employed to produce nanostructured composite powders with 5 and 15 wt% Ni. The powders were consolidated using spark plasma sintering (SPS) and conventional vacuum furnace sintering at temperatures of 1350 °C and 1450 °C. The composite powders were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and particle size measurements. The expansion/contraction of the compacted nanocomposite powders was studied through dilatometric analysis. The composite powders exhibited a uniform distribution of Ni, in both compositions. For the WC-5 wt% Ni nanocomposite powders, the average crystallite sizes for WC and Ni were 41.6 nm and 46.2 nm, respectively. While, for the WC-15 wt% Ni nanocomposite, the crystallite sizes for WC and Ni were 45.6 and 38.4 nm. The sintered composites were evaluated through XRD, SEM, measurements of density and Vickers hardness. The enhanced sinterability of the nanostructured powders enabled their consolidation into (WC-Ni) hard metal with densities approaching the theoretical relative density of 96.07 %, even with low binder content (5 wt% Ni). This is attributed to the uniform distribution of Ni and an extensive Ni-WC interface present prior to sintering. The SPS process at 1350 °C produced sintered bodies (WC-5 wt% Ni and WC-15 wt% Ni) with higher Vickers hardness (2322 HV and 1512 HV, respectively) and superior microstructural quality compared to samples produced by conventional vacuum furnace techniques. These results demonstrated that the mechanical properties of the system WC-Ni processed by SPS are superior to those obtained by vacuum furnace. Therefore, WC-Ni hard metals processed by this approach is a promising alternative to conventional hard metals (WC-Co) for a wide range of applications.

NiCo2O4-Nanoparticle-Catalyzed Microwave-Assisted Dehydrogenative Direct Oxidation of Primary Alcohols to Carboxylic Acids under Oxidant-Free Conditions

AbstractHere, we report the NiCo2O4-nanoparticle-catalyzed dehydrogenative direct oxidation of primary alcohols to carboxylic acid in the presence of KOH under microwave irradiation in the absence of any oxidant in good to excellent yields (75–99%) within a short reaction time (5–10 min). The polycrystalline cubic spinel phase of NiCo2O4 nanoparticles (NPs) with an average size of 25 nm were synthesized by the co-precipitation method and analyzed properly by using powder X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The NiCo2O4 NPs were stable under the reaction conditions and reused for up to eight cycles without appreciable loss in the yield of benzoic acid. According to mechanistic insight, the KOH acts as a second oxygen source and is essential for the synthesis of carboxylic acid from alcohols. The hydrogen gas was found to be the only byproduct of this method detected by chemical reactions.

Recycling of reduced graphene oxide from graphite rods in disposable zinc battery applicable to optical sensing

We report a low cost, simple method to fabricate reduced graphene oxide (rGO) optical sensors from recycled batteries. The rGO nanosheets were exfoliated by electrochemical method under the assistant of electric arc using 5 % KOH or NaOH electrolytes. The rGO nanosheets show interlayer distances ranging from 3.98 to 6.22 Å, and thicknesses from 0.62 to 6.00 nm, corresponding from 1 to 10 layers, respectively, determined from X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) measurements. Selected area electron diffraction (SAED) measurement indicates that rGO nanosheets content Miller-Bravais indices of (1–210) and (0–110) planes, and the hexagonal diffraction pattern of (0–110) plane of graphene sheets with d-spacing of 2.13 Å. Low defect densities in exfoliated rGO nanosheets were confirmed by ID/IG ratios ∼0.16 to 0.18 via Raman. The rGO nanosheets exfoliated in NaOH electrolyte have 19.35 % lower oxygen content compared to those exfoliated in KOH electrolyte inspected from XPS spectra. The performance of the rGO optical sensors is controlled by defects created from oxygen functional groups formed during exfoliation process. The responsivity and response (rise) time of NaOH-exfoliated rGO sensor are 0.46 A/W and 2.3 s, respectively. These values are 21 % (for responsivity) larger and 35 % (for rise time) smaller than those of KOH-exfoliated rGO sensor. The best-in-class of high gain were demonstrated in this study for the rGO optical sensors prepared from disposed graphite rods. The results demonstrate how disposed batteries can be recycled to produce photodetectors and other optoelectronic devices using cheap and relatively nontoxic methods.

Synthesis and color-tunable emission of ZnGaOx with different cation ratios by a polyacrylamide gel method

ZnGaOx nanoparticles with varied ratios of Zn2+ to Ga3+ were synthesized by a polyacrylamide gel route. The phase purity and structure are confirmed by X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectrometer (XPS). High crystal quality and elemental distribution are determined by transmission electron microscopy (TEM) measurements. The band gaps of the samples ranging from 3.22 to 4.47 eV are estimated by ultraviolet–visible (UV–Vis) diffuse refectance spectroscopy (DRS). Under the excitation of 325 nm laser light, ZnGaOx nanoparticles with the Zn2+/Ga3+ molar ratios of 0.8:2 and 0.7:2 show two emission peaks at 415 and 510 nm. A marked emission band at 695 nm is observed from the samples with Zn2+/Ga3+ molar ratios of 0.6:2, 0.5:2 and 0.4:2. The luminescence mechanism is discussed based on the experimental results. The luminescence color of as-synthesized samples gradually shifts from blue-green to green-yellow region with decreasing Zn2+/Ga3+ ratio. Interestingly, the sample with Zn2+/Ga3+ ratio of 0.7:2 has the CIE chromaticity coordinates of (x = 0.26, y = 0.33) close to the coordinates (x = 0.33, y = 0.33) of pure white light. Together, ZnGaOx phosphors with different emission colors without doping with rare earths have considerable potential applications in tunable light emitting diodes (LEDs) and color display.

Investigating Co and Fe Single-Atom (SA) Decorated NS-Doped Reduced Graphene Oxide (Co SA/NS-rGO, Fe SA/NS-rGO) Catalysts for NOx - to NH3 Pulsed Electroreduction

Electrochemical NH3 production has attracted much attention due to its potential to compensate for and supplement the Haber-Bosch process that produces hundreds of billions of kg of NH3 annually.1 One of the proposed electrochemical pathways to produce NH3 is the N2 reduction reaction. However, this reaction suffers from a high bond dissociation energy of 945 kJ mol−1, low NH3 Faradaic efficiency (F.E.) at high overpotentials, and NH3 quantification problems, limiting the extent of its applications.2,3 On the other hand, other N-containing species, NO3 - and NO2 -, that are commonly found in sewages and agricultural water, hold great potential for electrochemical NH3 production owing to their high NH3 yield rate and F.E. as well as building a self-sustaining NH3 production cycle working tandem with denitrification processes.1,4 In this study, we investigated Co and Fe single-atom (SA) decorated NS-doped reduced graphene oxide (Co SA/NS-rGO and Fe SA/NS-rGO) catalysts for nitrate reduction reaction (NO3RR) and nitrite reduction reaction (NO2RR). Both materials have defect-rich properties owing to preserving the nature of doped graphene materials even after the metal incorporation. The presence of the layered structure of graphene and homogenous distribution of Fe and Co SAs in the doped graphene basal plane is shown through High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) measurements. In addition to the Co and Fe SAs on the NS-rGO planes, SEM and X-ray Photoelectron Spectroscopy (XPS) investigations demonstrated that CoS and FeS could be simultaneously formed.We performed NO3RR and NO2RR experiments in 0.1 M KOH + 0.1 M KNO3 and 0.1 M KOH + 0.1 M KNO2 electrolytes with constant Ar purging in a glass H-Cell with a Nafion proton exchange membrane. We obtained NH3 with 65% F.E. at -0.4 V and a maximum of 2 mol s-1 molM -1 yield rate using Co SA/NS-rGO catalyst while with 60% F.E. at -0.5 V and 2.3 mol s-1 molM -1 yield rate using Fe SA/NS-rGO catalyst for NO3RR. Furthermore, the ∼10% NO2 - F.E. that is obtained for NO3RR experiments at -0.3 V and a significant increase in NH3 F.E. after -0.3 V for NO2RR indicates that NO3 - to NO2 - reduction takes place at this potential and our catalysts follow a two-step electron transfer mechanism. Moreover, linear sweep voltammetry (LSV) measurements indicated an onset potential of -0.4 V and -0.5 V for Co SA/NS-rGO and Fe SA/NS-rGO, respectively, for the NO3RR which are in excellent agreement with NO3RR F.E. results. We demonstrate that after prereduction at high negative potentials and with optimizations in experimental conditions, one can obtain >80% NH3 F.E. for NO3RR.To further increase the NH3 yield, three different pulsed electroreduction studies such as applying a potential at (1) the open circuit potential (OCP), (2) NO2 - reduction potential, (3) Co2+ or Fe2+ red-ox potentials and NH3 reduction potentials for 1-5 s intervals were performed. On Co SA/NS-rGO, an increased NH3 F.E. of ∼100% at -0.4 V was obtained under pulsed conditions. Also, by modulating NO3 - to NO2 - conversion to more negative potentials by pulsed electroreduction, significantly higher NH3 F.E. values were obtained for both catalysts. The higher NH3 F.E. and lower overpotential of Co SA/NS-rGO compared to Fe SA/NS-rGO could be attributed to the CoS formation that is more stable than FeS.

Synthesis of an Ultra-high Hardness Nanostructured AlSi10Mg Alloy via A Hybrid Laser Powder Bed Fusion/High-Pressure Torsion Approach

The hybrid combination of laser powder bed fusion (L-PBF) and high-pressure torsion, respectively an additive manufacturing (AM) and severe plastic deformation (SPD) has recently emerged as an important field of study due to the ability to produce novel nanostructured metals/alloys with simultaneous enhancements in mechanical (hardness, yield and tensile strengths) and functional (corrosion and wear) performances. Pioneering studies on the hybrid L-PBF/HPT approach was focused on 316L stainless steel (316L SS) and AlCuMg alloys due to their widespread engineering applications. AlSi10Mg, an alloy widely used in the automotive industry is expected to benefit from this hybrid combination but has not been investigated before. Thus, this study aims to investigate the microstructural features and hardness of the nanostructured AlSi10Mg attained by the hybrid L-PBF/HPT technique via transmission electron microscopy (TEM) and Vickers microhardness (HV) measurements, respectively. The results showed novel microstructures, including nano-scale grain sizes (< 200 nm), nano-sized Mg2Si precipitates (~200 nm), and dense dislocation networks. The average hardness value was determined as 230 ± 20 HV, homogeneously distributed throughout the disk surface as indicated by the low deviation (error bar). The microstructures of L-PBF/HPT-synthesised AlSi10Mg are different and significantly refined than conventionally cast AlSi10Mg, which contributed to the two- to four-fold hardness (typical HV values of as-cast AlSi10Mg ranges from 60 – 68 HV) increase via grain boundary and dislocation strengthening mechanisms.

Heteroepitaxial MOF-on-MOF Photocatalyst for Solar-Driven Water Splitting.

Assembly of different metal-organic frameworks (MOFs) into hybrid MOF-on-MOF heterostructures has been established as a promising approach to develop synergistic performances for a variety of applications. Here, we explore the performance of a MOF-on-MOF heterostructure by epitaxial growth of MIL-88B(Fe) onto UiO-66(Zr)-NH2 nanoparticles. The face-selective design and appropriate energy band structure alignment of the selected MOF constituents have permitted its application as an active heterogeneous photocatalyst for solar-driven water splitting. The composite achieves apparent quantum yields for photocatalytic overall water splitting at 400 and 450 nm of about 0.9%, values that compare much favorably with previous analogous reports. Understanding of this high activity has been gained by spectroscopic and electrochemical characterization together with scanning transmission and transmission electron microscopy (STEM, TEM) measurements. This study exemplifies the possibility of developing a MOF-on-MOF heterostructure that operates under a Z-scheme mechanism and exhibits outstanding activity toward photocatalytic water splitting under solar light.

Edge-Passivated Monolayer WSe2 Nanoribbon Transistors.

The ongoing reduction in transistor sizes drives advancements in information technology. However, as transistors shrink to the nanometer scale, surface and edge states begin to constrain their performance. 2Dsemiconductorsliketransition metal dichalcogenides (TMDs)have dangling-bond-free surfaces, hence achieving minimal surface states. Nonetheless, edge state disorder still limits the performance of width-scaled 2D transistors. This work demonstrates a facile edge passivation method to enhance the electrical properties of monolayer WSe2 nanoribbons, by combining scanning transmission electron microscopy, optical spectroscopy, and field-effect transistor (FET) transport measurements. Monolayer WSe2 nanoribbons are passivated with amorphous WOxSey at the edges, which is achievedusing nanolithography and a controlled remote O2 plasma process. The same nanoribbons, with and without edge passivation are sequentially fabricated and measured. The passivated-edge nanoribbon FETs exhibit 10 ± 6 times higher field-effect mobility than the open-edge nanoribbon FETs, which are characterized with dangling bonds at the edges. WOxSey edge passivation minimizes edge disorder and enhances the material quality of WSe2 nanoribbons. Owing to its simplicity and effectiveness, oxidation-based edge passivation could become a turnkey manufacturing solution for TMD nanoribbons in beyond-silicon electronics and optoelectronics.

Unusual negative thermal expansion of inter-cluster –C–BC–C– bond in carbon rich boron carbide observed using in situ x-ray diffraction technique

Temperature dependent bonding behavior plays a significant role in deciding properties of high temperature ceramics like boron carbide. However, few studies to date have addressed the physical properties of this class of materials with respect to their temperature dependent bonding nature. In addition, materials with the flexibility to accommodate variations in interatomic bonding and lattice vibrations over a wide range of temperatures are less known. In this work, temperature dependent structural analyses of carbon-rich boron carbide microflakes using in situ powder x-ray diffraction techniques (up to 1000 °C) supported by transmission electron microscopy measurements reveal that while most bonds in the rhombohedral structure increase in length with temperature; there is no change in certain bond lengths. However, there is an unusual decrease in length (∼1.03%) of the inter-cluster –C–(central boron)BC–C– without any polyhedral redistribution. This is accompanied by an increase in lattice vibrations without significant alteration to the crystal structure over the wide temperature range studied. Temperature dependent micro-Raman experiments further confirmed the above observations. The above bonding behavior could be directly correlated to the trends in reported results of high temperature conductivity via the model of hole hopping through specific atomic positions of the rhombohedral framework, thus opening the scope to investigate structure–property relationships in high temperature functional materials.

Ultrasonic Preparation of Nano-CaCO3 Templates and Hollow Mesoporous SiO2 Nanoparticles for Voriconazole Loading

CaCO3 nanoparticles (nano-CaCO3) as nano-templates were prepared using CaCl2 and Na2CO3 solutions under controlled sonication (19.5 kHz). Using the same ultrasonic device, subsequently, hollow mesoporous silica nanoparticles (HMSNs) were obtained by the hard template of nano-CaCO3. HMSNs were selected as carriers for the antifungal drug voriconazole (VOR) loading to overcome poor water solubility. Three-dimensional CaCO3 nanosheets HMSNs were obtained under gentle sonication. Three-dimensional CaCO3 nanosheets of 24.5 nm (hydrodynamic diameter) were obtained under 17.6 W for 3 min. HMSNs were synthesized by double-template method with nano-CaCO3 as the hard template. Transmission electron microscopy measurements showed that the prepared HMSNs possess hollow structures with particle size between 110 and 120 nm. Nitrogen physisorption at -196 °C revealed that the HMSNs had high surface area (401.57 m2/g), high pore volume (0.11 cm3/g), and uniform pore size (2.22 nm) that facilitated the effective encapsulation of VOR in the HMSNs. The loading capacity of VOR (wt%) on the HMSNs was 7.96%, and the total VOR release amount of VOR-HMSNs material was 71.40% at 480 min. The kinetic model confirmed that the release mechanism of HMSNs nanoparticles followed Fickian diffusion at pH = 7.4 and 37 °C. Moreover, the cumulative VOR release at 42 °C (86.05%) was higher than that at 37 °C (71.40%). The cumulative release amount of VOR from the VOR-HMSNs material was 92.37% at pH = 5.8 at the same temperature. Both nano-CaCO3 templates and HMSNs were prepared by sonication at 19.5 kHz. The as-prepared HMSNs can effectively encapsulate VOR and released drug by Fickian diffusion.Graphical Nano-CaCO3 Templates and Hollow Mesoporous SiO2 Nanoparticles

Synthesis of MS2 (M=Mo, W) thin films by chemical solution deposition

MoS2 and WS2 as typical two-dimensional transition metal dichalcogenides have been investigated since of rich physicochemical properties. To deposit MoS2 and WS2 thin films, chemical solution deposition as a facile route had been used showing the advantages of high component controllability, low cost, sequential processing of device, easy preparation of large-size films and facile preparation of films on irregular substrates. Here, Mo1-xWxS2 (0 ≤ x ≤ 1) thin films were successfully prepared by chemical solution deposition method. The phases as well as microstructures were investigated using field emission scanning electron microscope, atomic force microscopy, transmission electron microscopy, Raman and x-ray photoelectron spectroscopy measurements. The prepared Mo1-xWxS2 thin films are used as hydrogen evolution reaction (HER) catalyst electrodes for water splitting. The Mo0·3W0·7S2 thin film catalyst shows an overpotential of 345 mV at a current density of 10 mA cm−2 and the corresponding Tafel slope of 169 mV dec−1 in 0.5 M sulfuric acid. The results will provide a facile method to deposit MX2 thin films.

Swift Au+9 ion irradiation-induced defects and alloy complexity effect on the mechanical hardness of NiCoCrFePd HEA and NiCoCrFe MEA

The outstanding radiation damage stability of an NiCoCrFePd high entropy alloy (HEA) as compared to conventional alloys poses the question for the mechanism of an ion–matter interaction. The positron annihilation lifetime spectroscopic and TEM (transmission electron microscopic) measurements are implemented to trace different kinds of defects produced by 120 MeV Au+9 ion irradiation and their evolution as a function of ion fluence. The variation of lifetimes and corresponding intensities with the ion fluence indicates the formation of dislocation-type defects at a lower ion fluence and vacancy clusters at a higher ion fluence caused by coalescence or agglomeration of dislocation defects. Formation of different types of defects in turn modulates the strain development inside the crystal. Additionally, the HR-TEM investigation of NiCoCrFePd HEA also exhibits the formation of dislocation and vacancy clusters with the average size of vacancy clusters increases from ∼2.9 ± 0.1 to ∼3.8 ± 0.1 nm with the increases in the ion fluence. Surprisingly, the average defect cluster size in NiCoCrFePd HEA is suppressed compared to NiCoCrFe MEA, thereby showing the enhanced radiation stability on Pd incorporation due to the high defect recombination caused by reduced thermal conductivity and high lattice distortion. Nano-indentation measurement shows that the radiation hardening behavior of the NiCoCrFePd HEA responded slowly owing to its damage suppression property as compared to the NiCoCrFe MEA. Additionally, softening behavior also appeared at an early fluence in NiCoCrFe MEA compared to the NiCoCrFePd HEA signifying its excellent resistance to defect accumulation.

Potential application of novel Janus amphiphilic nanoparticles to improve the viscoelasticity of polymer solution for enhanced oil recovery

Nanomaterials were widely employed to improve the physicochemical properties of various chemicals in enhanced oil recovery (EOR). In the present study, the novel Janus amphiphilic silica nanoparticle (JNS) was successfully prepared, whose structure and property were confirmed via Fourier transform infrared spectroscopy, Zeta potential measurement, transmission electron microscopy, interfacial tension and contact angle measurements. Then the effects of JNS, hydrophilic silica nanoparticles (HLS) and hydrophobic silica nanoparticles (HBS) on the viscosity of the hydrophobic modified HPAM (HOA101) solution were studied. It was found that JNS exhibited a greater ability to enhance the viscosity, temperature resistance and salt resistance of HOA101 solution in comparison to the other two polymer nanofluids. The interaction mechanisms between nanoparticles and HOA101 were proposed, which were verified by the viscoelasticity measurements and control experiments. Core flooding experiments and micromodel flooding tests demonstrated that the JNS/HOA101 dispersion could effectively expand the sweep efficiency to highly increase the oil recovery from 8.4% to 14.3%.

Synthesis and characterization of silver nanoparticles by electrochemical method

In this work, silver nanoparticles (Ag-NP) were synthesized by electrochemical method, using a metallic silver cathode and anode in deionized water. The polarity was inverted every 60 seconds generating a voltage variation in order to obtain three different concentrations of Ag nanoparticles: 16 ppm, 24 ppm and 34 ppm. The synthesized nanoparticles were structurally, optically and morphologically characterized by XRD, UV-Vis and Transmission Electron Microscopy (TEM), respectively. XRD patterns showed the peaks at 38.11° and 64.42° from Ag. The surface plasmon resonance peak in absorption spectra on silver colloidal solution showed absorption from 422 nm to 429 nm. TEM measurement provided an average particle size for the concentrations of 16 ppm, 24 ppm and 34 ppm of 4.23 nm, 3.95 nm and 3.27 nm, respectively. The test with the E. coli bacteria showed that there is an inhibition in the growth of the bacteria with the presence of silver nanoparticles.

The feasibility of an ultrathin dual-barrier scheme to inhibit interfacial diffusion and reactions in contact stacks of Co/NiSi/Si

Self-assembled monolayers (SAMs) and self-forming barriers (SFBs) are highly effective barriers of copper (Cu) interconnect systems. However, the barrier capability of SAMs and SFBs to contact metallization of cobalt (Co), a new important interconnect material, is yet to be explored. Here, contact layers of NiSi are grown by a two-step silicidation process on Si. Then, an all-wet SAM-seeding and electroless-plating process is used to coat (in sequence) an amine SAM and a MnO-added (only 0.1%) Co film onto NiSi/Si, using a specifically formulated N2H4-CoSO4-MnSO4 bath. The thermal stability of the Co(MnO)/SAM/NiSi/Si is evaluated, using (unalloyed) Co/SAM/NiSi/Si as a control. The 1.3 nm-thin SAM cannot protect the Co/SAM/NiSi/Si from catastrophic failure upon annealing, associated with serious Co/Si interfacial diffusion and the formation of faceted CoSi crystallites and a layered Co2Si/CoSi2 nanostructures at specific locations. By contrast, the Co(MnO)/SAM/NiSi/Si maintains an intact stack after annealing. Collaborative research results generated from (scanning) transmission electron microscopy, nanoscratch testing and surface-energy measurements suggest that the thermal stability enhancement of the whole Co(MnO)/SAM/NiSi/Si contact stack is related to the interfacial segregation of the added MnO as sub-5-nm dispersive crystalline particles, giving multiple functionalities as a SFB, Co-grain strengthening agent, and wetting/adhesion promoter.

Palladium nanoparticles immobilized on TiO2 nanosheets matrix for the valorization of furfural to produce tetrahydrofurfuryl alcohol

The development of highly dispersed and ultra-small sized metal nanocatalysts are crucial for the hydrogenation of biomass. In this work, the ultrafine palladium nanoparticles modified on the 2-dimensional TiO2 nanosheets (PdNPs-TiO2NSs) as a catalyst for the selective hydrogenation of furfural (FAL) to tetrahydrofurfuryl alcohol (THFA) was achieved. The as-prepared PdNPs-TiO2NSs catalyst was characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The spherical PdNPs were well dispersed on TiO2NSs surface with the average particle size of 1.6 ± 0.2 nm was confirmed by TEM measurements. The PdNPs-TiO2NSs catalyst showed the remarkable hydrogenation of FAL to THFA by 100 % of conversion with > 99 % of selectivity towards THFA at 140 °C and 20 bar hydrogen pressure in the 4 h reaction time. The stability and reusability test for PdNPs-TiO2NSs were found to be good even after three catalytic cycles with high catalytic activity.

Characterization of Mass, Diameter, Density, and Surface Properties of Colloidal Nanoparticles Enabled by Charge Detection Mass Spectrometry.

A variety of scattering-based, microscopy-based, and mobility-based methods are frequently used to probe the size distributions of colloidal nanoparticles with transmission electron microscopy (TEM) often considered to be the "gold standard". Charge detection mass spectrometry (CDMS) is an alternative method for nanoparticle characterization that can rapidly measure the mass and charge of individual nanoparticle ions with high accuracy. Two low polydispersity, ∼100 nm diameter nanoparticle size standards with different compositions (polymethyl methacrylate/polystyrene copolymer and 100% polystyrene) were characterized using both TEM and CDMS to explore the merits and complementary aspects of both methods. Mass and diameter distributions are rapidly obtained from CDMS measurements of thousands of individual ions of known spherical shape, requiring less time than TEM sample preparation and image analysis. TEM image-to-image variations resulted in a ∼1-2 nm range in the determined mean diameters whereas the CDMS mass precision of ∼1% in these experiments leads to a diameter uncertainty of just 0.3 nm. For the 100% polystyrene nanoparticles with known density, the CDMS and TEM particle diameter distributions were in excellent agreement. For the copolymer nanoparticles with unknown density, the diameter from TEM measurements combined with the mass from CDMS measurements enabled an accurate measurement of nanoparticle density. Differing extents of charging for the two nanoparticle standards measured by CDMS show that charging is sensitive to nanoparticle surface properties. A mixture of the two samples was separated based on their different extents of charging despite having overlapping mass distributions centered at 341.5 and 331.0 MDa.

Giant Spin‐Charge Conversion in Ultrathin Films of the MnPtSb Half‐Heusler Compound

AbstractHalf‐metallic half‐Heusler compounds with strong spin‐orbit‐coupling and broken inversion symmetry in their crystal structure are promising materials for generating and absorbing spin‐currents, thus enabling the electric manipulation of magnetization in energy‐efficient spintronic devices. In this work, the spin‐to‐charge conversion in the sputtered half‐Heusler MnPtSb within thickness (t) range from 1 to 6 nm is reported. A combination of X‐ray and transmission electron microscopy measurements evidence the epitaxial nature of these ultrathin non‐centrosymmetric layers, with a clear (111)‐orientation on top of (0001) single‐crystal sapphire. By broadband ferromagnetic resonance (FMR), a four orders of magnitude tunable spin accumulation in the MnPtSb‐based heterostructures, within t = 1–6 nm range, is observed. By using spin pumping FMR, a remarkable t‐dependent spin‐charge conversion in the MnPtSb layers is measured, which clearly demonstrates the interfacial origin of the conversion. When interpreted within the inverse Edelstein effect (IEE), the spin‐charge conversion efficiency extracted at room temperature for the thinnest MnPtSb layer reaches λIEE≈3 nm, representing an extremely high conversion. The still never explored ultrathin regime of the MnPtSb films studied in this work and the discovery of their outstanding functionality are two ingredients that demonstrate the potentiality of such materials for future applications in spintronics.

Optical damage thresholds of single-mode fiber-tip spintronic terahertz emitters.

Spintronic terahertz emitters (STEs) are gapless, ultrabroadband terahertz sources that can be driven within a wide pump-wavelength and repetition-rate range. While STEs driven by strong pump lasers operating at kilohertz repetition rates excel in generating high electric field strengths for terahertz spectroscopy or ellipsometry, newly advancing technologies such as ultrafast modulation of terahertz polarization, scanning tunneling microscopy, laser terahertz emission nanoscopy, and fully fiber-coupled integrated systems demand an STE pumping at megahertz repetition rates. In all these applications the available terahertz power is ultimately limited by the STE's optical damage threshold. However, to date, only very few publications have targeted this crucial topic and investigations beyond the kilohertz repetition-rate regime are missing. Here, we present a complete study of our single-mode fiber-tip STEs' optical damage thresholds covering the kilohertz, megahertz, and gigahertz repetition-rate regimes as well as continuous-wave irradiation. As a very important finding, we introduce the necessity of classifying the optical damage threshold into two regimes: a low-repetition-rate regime characterized by a nearly constant fluence threshold, and a high-repetition-rate regime characterized by an antiproportional fluence dependence ("average-power threshold"). For our single-mode fiber-tip STEs, the transition between these regimes occurs around 4 MHz. Moreover, we present a cohesive theory of the damaging thermodynamical processes at play and identify temperature-driven inter-layer diffusion as the primary cause of the STE failure. These findings are substantiated by atomic force microscopy, infrared scattering-type scanning near-field optical microscopy, and scanning transmission electron microscopy measurements. This new level of understanding offers a clear optimization lever and provides valuable support for future advancements in the promising field of spintronic terahertz emission.